1. Field of the Invention
The present invention relates to solar cells.
2. Description of the Prior Art
CuInSe2 (which may be referred to as CIS in the following) and Cu(In, Ga)Se2 (which may be referred to as CIGS in the following), which is obtained by substituting a part of In of the CIS with Ga, are known as compound semiconductors (of chalcopyrite structure) comprising at least one element from each of groups Ib, IIIb and VIb. Conventionally, thin-film solar cells using these semiconductors as the light-absorption layer have been under research. These thin-film solar cells have a high energy conversion efficiency, which does not deteriorate due to exposure to light or the like. Therefore, such a thin film solar cell has received attention.
In the solar cell using CIS or CIGS as the light-absorption layer, the solar cell can have high efficiency by using a window layer made of CdS. This is because the conduction band offset between CdS and CIS (or CIGS) is suitable for production of high efficiency solar cells. Therefore, in order to obtain high efficiency solar cells, it is preferable to use a semiconductor having a band structure similar to that of CdS and CIS (or CIGS) for the window layer.
However, when forming the window layer using a semiconductor having a band structure similar to that of CdS, the semiconductor layer serving as the light-absorption layer is damaged so that the efficiency is reduced.
Therefore, with the foregoing in mind, it is an object of the present invention to provide a solar cell having a band structure that can achieve a high conversion efficiency with reduced damage to the semiconductor layer serving as the light-absorption layer.
A solar cell of the present invention includes a first semiconductor layer that is p-type, and a second semiconductor layer that is n-type formed over the first semiconductor layer. The solar cell includes a layer A made of an insulator or a semiconductor different from the first semiconductor layer and the second semiconductor layer between the first semiconductor layer and the second semiconductor layer. The band gap Eg1 of the first semiconductor layer and the band gap Eg2 of the second semiconductor layer satisfy the relationship Eg1 less than Eg2. The electron affinity "khgr"1 (eV) of the first semiconductor layer and the electron affinity "khgr"2 (eV) of the second semiconductor layer satisfy the relationship 0xe2x89xa6("khgr"1xe2x88x92"khgr"2) less than 0.5. The average layer thickness of the layer A is 1 nm or more and 20 nm or less. This solar cell has a band structure that can provide a high conversion efficiency. Furthermore, since the solar cell is provided with the layer A, the first semiconductor layer serving as the light-absorption layer can be prevented from being damaged when forming the second semiconductor layer. Therefore, with this solar cell, a high efficiency solar cell can be achieved.
In the above solar cell, the second semiconductor layer may be formed by sputtering. This embodiment allows the second semiconductor layer having various compositions to be formed.
In the above solar cell, the second semiconductor layer may be formed of an oxide containing Zn and Mg. This embodiment facilitates formation of a band structure that can provide a high efficiency. In this embodiment, in particular, it is preferable that the oxide is represented by a general formula Zn1xe2x88x92XMgXO, where 0 less than X less than 0.5.
In the above solar cell, the layer A may include Cd and S as main components (constituent elements) or may include Zn, O and S as main components (constituent elements). This embodiment provides a satisfactory junction. In this embodiment, it is preferable that the layer A is formed from a solution containing a salt of Cd or Zn and a sulfur-containing compound. This embodiment allows the layer A to be formed without causing damage to the first semiconductor layer.
In the above solar cell, the layer A may be formed of a compound containing Se and at least one element selected from Zn and In. This embodiment provides a satisfactory junction.
In the above solar cell, it is preferable that the first semiconductor layer includes at least one element from each of groups Ib, IIIb and VIb (constitution 1). In the case of this constitution, it is preferable that the element from group Ib is Cu, the element from group IIIb is at least one element selected from In and Ga, and the element from group VIb is at least one element selected from Se and S. This embodiment provides a solar cell having a particularly high efficiency. Throughout this specification, xe2x80x9cgroups Ib, IIIb, and VIbxe2x80x9d refer to xe2x80x9cgroups 1B, 3B, and 6Bxe2x80x9d of the periodic table of elements according to the old IUPAC recommendation before 1985.
In the case of the constitution 1, it is preferable that the solar cell further includes a third semiconductor layer disposed between the first semiconductor layer and the layer A. It is preferable that the third semiconductor layer includes an element from group Ib, an element from group IIIb and sulfur, and an atomic ratio of sulfur in the third semiconductor layer is higher than that in the first semiconductor layer. This embodiment provides a satisfactory junction.
In the case of the constitution 1, it is preferable that the first semiconductor layer comprises at least one element selected from Cd and Zn on the surface thereof on the side of the layer A. This embodiment provides a satisfactory junction.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.